On 17/10/2016 16:50, Radim Krčmář wrote:
> 2016-10-17 07:47-0200, Marcelo Tosatti:
>> On Fri, Oct 14, 2016 at 06:20:31PM -0300, Eduardo Habkost wrote:
>>> I have been wondering: should we allow live migration with the
>>> invtsc flag enabled, if TSC scaling is available on the
>>> destination?
>>
>> TSC scaling and invtsc flag, yes.
>
> Yes, if we have well synchronized time between hosts, then we might be
> able to migrate with a TSC shift that cannot be perceived by the guest.
>
> Unless the VM also has a migratable assigned PCI device that uses ART,
> because we have no protocol to update the setting of ART (in CPUID), so
> we should keep migration forbidden then.

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We don't publish the ART leaf at all, do we?
>> 1) Migration: to host with different TSC frequency.
>
> We shouldn't have done this even now when emulating anything newer than
> Pentium 4, because those CPUs have constant TSC, which only lacks the
> guarantee that it doesn't stop in deep C-states:
Right, but:
>> since Linux guests use kvmclock and Windows guests use Hyper-V
>> enlightenment, it should be fine to disable 2).
... and 1 too.
We should also blacklist the TSC deadline timer when invtsc is not
available.
Paolo